Geological characteristics and exploration potential of deep coalbed methane in the slope area of the northeastern Ordos Basin

In recent years, significant breakthroughs have been achieved in the exploration of deep coalbed methane (CBM) in the Daniudi area, northern Ordos Basin. This study aims to determine the potential for deep CBM exploration in this area. Based on regional geology and data derived from core and maceral observations, scanning electron microscopy (SEM) image analysis, proximate analysis, mercury injection capillary pressure (MICP) analysis, lowtemperature liquid-nitrogen adsorption measurements, and CO₂ adsorption and methane isothermal adsorption experiments, along with on-site gas content tests, we delve into the geological characteristics of deep coal seams in the Daniudi area in terms of their spatial distribution, lithotypes, coal quality, reservoir properties, and gas-bearing properties. The No.8 coal seam of the Taiyuan Formation is identified as the dominant coal reservoir in the Daniudi area. The results indicate that this coal seam is consistently distributed throughout the Daniudi area, with a thickness ranging from 2 to 14 m and vitrinite reflectance (R_o) of 1.7 %. Bright to semi-bright coals occur in the upper part of the coal seam as featuring low ash content, indicating high coal quality and providing a material basis for CBM accumulation. The No.8 coal seam shows well-developed micropores and macropores, with micropores being predominant, accounting for 67.5 %. The bright to semi-bright coals with low ash content therein display high total pore volume, specific surface area of pores,micropore proportion, and porosity. The roof of the coal seam contains thickly laminated limestones and dark mudstones, as characterized by simple structure and low formation water content, suggesting a dry-coal gas-bearing system. With total gas content ranging from 20.3 to 47.1 m³/t and free gas proportion from 33.2 % to 66.2 %, the No.8 coal seam is rich in both adsorbed and free gases. This coal seam manifests a gas-bearing area of 2003 km², CBM resources of 5422 × 10⁸ m³, and CBM resource abundance of 2.71 × 10⁸ m³/km². These findings indicate that deep coals in the Ordos Basin are characterized by favorable geological condition for CBM generation, representing a significant exploration and development target.